Injection blow molding machines apply a parison to a core rod by injecting molten plastic material into the cavity of an injection mold into which a core rod extends. The plastic material coats the core rod and fills the mold cavity clearance that exists between the outside surface of the core rod and the wall of the injection cavity.
In accordance with standard operating procedure, the pressure on the injected plastic material is then reduced and a negative pressure or suction is then applied to draw back some of the plastic from the runners of the injection mold.
The wall of each injection cavity is cooled by the circulation of cooling fluid through the cavity walls and this cooling causes the parison to cool and shrink away from the cavity walls.
When the core rod and parison are transferred to a blow molding cavity, it is essential that the parison release easily and uniformly from the surface of the core rod in order to blow an object of the desired uniform wall thickness. In spite of the compounding of the plastic material and the smooth surfaces on the core rods, parisons do stick to core rods on occasions and this can cause defective articles. Such results require inspection of the output of the blow molding machine and result in wasted material, time, and production.
This invention provides a blowing fluid under pressure which is applied through the core rod and into the interior of the parison while the parison is still located in the cavity of the injection mold. Since the cavity is filled with molten plastic by the injection operation, there is no space into which the parison can expand in response to the application of fluid pressure to the inside of the parison, except such clearance as develops as the result of shrinkage of the plastic in the cavity. This shrinkage occurs as the molten plastic begins to cool.
Plastics have a high coefficient of thermal expansion; and when pressure is maintained within the parison during the shrinkage of the plastic material of the parison, the parison remains in contact with the wall of the injection mold cavity and shrinks away from the surface of the core rod.
Although, theoretically, the blowing fluid may push the parison away from the entire surface area of the core rod, the core rod is the only support for the parison as it is removed from the injection mold and shifted to a blowing mold or some other treating station. Thus the parison remains in contact with the top portion of the core rod during its transport to the next performance station; but the blowing operation which causes shrinkage away from the core rod leaves the parison loose on the core rod as it travels to the blowing station or other performance station.
Since any sticking of the parison to the core rod is eliminated more uniform results are obtained in the subsequent blowing operation.
Another advantage is that the parison remains in contact with the cooled wall of the injection mold cavity as the parison shrinks; and the air which is injected into the parison insulates the parison from the core rod which is at high temperature. The parison remains in contact with the core rod for a shorter time and this reduces heat buildup in the core rods.
One of the limitations on the speed at which injection blow molding apparatus can operate is the building up of heat in the core rods. By reducing heat buildup in the core rod, this invention permits the blow molding apparatus to operate at higher production speeds.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.